1. Field of the Invention
This invention relates generally to apparatus and method used to measure a film thickness. Particularly, this invention relates to a novel apparatus and method to perform rapid high-resolution measurements of film thickness and thickness uniformity on a semiconductor wafer.
2. Description of the Prior Art
Precise control and measurement of thin film thickness has become a challenge to those of ordinary skill in the art of integrated circuit (IC) manufacture. Particularly, as smaller and denser device geometry on integrated circuit (IC) chips are now built by the microelectronics industry for achieving increasing amounts of computing power. Specifically, the advent of denser, larger-scale integration has placed greater demand on the precise measurement of thin film thickness of a polysilicon layer. As the metal-oxide-field-effect transistor (MOSFET) is now favored over the bipolar transistor among the devices used in ICs, there are practical advantages, in most cases, to making the “metal” electrode in the MOS device and interconnection “wires” of polysilicon. Critical to the development, production and final performance of advanced IC's is the precise control of the polysilicon layer thickness and the doping density. The polysilicon layer is typically sandwiched between two SiO2 layers, i.e., a thermal SiO2 layer and a deposited SiO2 inter-metal dielectric layer. The process monitoring and control is provided by post-fabrication metrology, performed outside the thin film deposition tools, for the evaluation of film thickness, doping density, uniformity, and defects.
There are several conventional methods of carrying out the processes of determining the film thickness by analyzing the light reflected from the film applying the measurement techniques of ellipsometry and interferomitry. The interferometer measurements utilize the partial reflections are generated when light passes between media with differing indices of refraction. When the thickness of film has a range of few wavelengths of light, an interference pattern is generated from interference between the light reflected from the top surface and the light reflected from the bottom surface. Analyses of these interference patterns generated from constructive and destructive interference at different wavelengths of light provide information related to the thickness of the film. When the refractive index is known, the film thickness can be determined through analyses of the interference patterns. FIG. 1 is a diagram showing the reflections of light from the top and bottom surfaces of a silicon wafer covered by a thin film upon which the light is projected for the purpose of determining a film thickness. The incident beam I is projected to the silicon wafer covered with a thin film and there are two reflected beams, i.e., R1 reflected from the top surface and R2 from the bottom surface according to Fresnel's formula. By applying a highly coherent light and by ignoring smaller internal reflections, effects of constructive and destructive interference can be observed. As the beam R2 travels additional optical path of 2nT than R1 where n is the refraction index of the film and T is the film thickness, a phase shift of the light is produced due to the optical path difference. The phase shift represented by Δφ between two paths is a function of a specific wavelength λ and film thickness T, i.e., Δφ=4πT/λ. By examining the patterns of interference between the reflected beam from the top surface of the film and the surface underneath the film, the thickness of the film can be determined. Different techniques of film thickness determinations are disclosed in various Patents such as U.S. Pat. Nos. 5,392,118, 5,403,433, 5,469,361, 5,587,792, and 5,604,581.
In addition to detecting the film thickness as discussed above, for the purpose of semiconductor manufacture, it is often desirable to determine the variations of film thickness over the surface of a silicon wafer. Conventional method of measuring the thickness variations are accomplished by placing the wafer on a motorized stage under an interferometer and positioning the wafer at a set of points on the wafer surface and carrying out a thickness measurement at each point. This method involves a start and stop of the stage wafer motion and thickness measurement by scanning a range of wavelengths at each point. Due to the operation requirements and length of time necessary to control the wafer stage motions and thickness determination measurements, conventional method can only be applied to measure the thickness at few points on the surface of the wafer surface typically 5, 13, and 49 points are measured. The thickness measurements made on these points are then presented as a contour map based on the data obtained from these points. As higher circuit densities are now formed on the silicon wafer, variation of film thickness measurements on 49 or even few hundred points over the entire wafer is gradually becoming insufficient. Higher resolution is required for measuring the thickness variations over the wafer surface to assure high quality of wafers are used to make integrated circuits with very high circuit density.
However, for those of ordinary skill in the art, improvement of film thickness measurement resolution by making the thickness measurement at more points is difficult because the number of required measurement points increases as the square of the increased density. The time required for motion control operations and scanning the range of wavelengths at each point as required for thickness determination by applying the reflection interference techniques described above grows linearly with the number of points measured and as the density squared. Therefore, a need still exists in the art to provide a new and improved technique to conduct film thickness variation measurement that can be practically carried out at higher resolution over the wafer surface to satisfy the requirement of modern ultra-high density integration now imposed on semiconductor industry.